1. Technical Field
The invention relates to lithium cells, and in particular to methods for forming lithium ion electrodes having graphitic carbon bases for use as an anode in a lithium cell.
2. Background Art
Lithium cells are being developed as replacements for conventional batteries, particularly for applications where high voltage, high capacity, low weight, and long shelf life is required. Moreover, unlike some conventional batteries, such as nickel cadmium batteries, which include component materials that may adversely affect the environment, lithium battery cells use component materials which are substantially less harmful.
Although lithium cells have numerous advantages over conventional batteries, the high chemical reactivity of lithium has caused problems in the fabrication and safe operation of the cells. For example, lithium is highly reactive with water and, if brought into contact with water, will ignite. Lithium can also be unstable when in contact with cell electrolytes. Moreover, after prolonged use, lithium cells can become unsafe due to the formation of lithium dendrites which can cause shorting within the cell.
Conventionally, the cell includes a pure lithium anode. To overcome problems associated with pure lithium electrodes, it has been proposed to replace the conventional electrode base material, with an alternate material having a greater stability with respect to the cell electrolytes. To be effective, the alternate material should exhibit low equivalent weight and low voltage versus lithium. Carbon is a candidate for alternate anode material since it possesses the above-mentioned properties. To function as the anode of a lithium cell, lithium ions must be incorporated into the carbon, thereby producing a carbon/lithium-ion anode.
Although some success has been achieved in developing carbon/lithium-ion anodes, problems remain which have heretofore prevented the attainment of the full advantages of carbon/lithium-ion anodes. Generally, a carbon/lithium-ion anode is fabricated by first forming a carbon electrode from graphitic carbon, then reacting the carbon electrode with lithium to cause lithium ions to be incorporated into the graphitic carbon. Problems occur in both fabrication steps. To properly allow for subsequent reaction with lithium, a carbon material having an optimal surface area must be employed. If either too great or too little carbon surface area is available, an effective lithium ion electrode can not be formed. Moreover, graphitic carbon particles must be tightly bound together to prevent the particles from dissolving or dispersing once placed within an electrolyte bath. One fabrication method involves compacting the particles at high temperatures and pressures. However, sintering occurring at the high temperatures and pressures renders the carbon electrode less desirable and causes changes to the carbon surface properties which prevent subsequent reaction with lithium. Accordingly, an ambient temperature fabrication method is preferred. For ambient temperature fabrication, a binder material must be provided to hold the carbon particles together. However, if too much binder is provided with the graphitic carbon, low specific energy, poor rate capability and inadequate lithium reaction occurs. Conversely, if an insufficient amount of binder is provided, the graphitic carbon particles tend to disperse or dissolve within the electrolyte bath. In general, the ratio of the binder material weight to the total carbon surface area is an important factor.
Once a suitable carbon electrode is formed, difficulties arise in incorporating lithium ions into the carbon anode. To incorporate lithium ion, a carbon electrode is typically immersed within an electrolyte bath with a lithium ion source which may be a lithium-containing electrode such as a piece of lithium metal, lithiated titaninum disulfide TiS.sub.2, or lithiated cobalt oxide. A current is applied between the lithium source and carbon electrodes. Lithium ions, drawn from the lithium source, react with the carbon electrode. To achieve reversible incorporation of the lithium ions into the carbon, the carbon must become intercalated with lithium ions, i.e., lithium ions must become loosely bonded between layers of carbon atoms within the graphitic carbon crystals. It has been found that conventional techniques for reacting lithium ions with graphitic carbon do not achieve reversible intercalation. Rather conventional techniques merely achieve a surface reaction between the lithium ions and the carbon electrode. Carbon electrodes which have undergone only a surface reaction with lithium ions do not allow reversible reactions with lithium ions. Hence, a lithium battery cell incorporating such a non-reversible carbon electrode is not rechargeable.
Heretofore, no effective techniques have been developed for forming carbon electrodes and for subsequently reacting the carbon electrodes with lithium to properly intercalate the electrode with lithium ions.